Summaries

What Did Dickinson Research Find?

On Wednesday morning, as soon as the news broke that TMI was releasing radioactivity, two professors in the Department of Physics and Astronomy made preparations for taking data.Both professors,
John Luetzelschwab and Priscilla Laws, had degrees in nuclear
physics and Luetzelschwab had been doing research on radioactivity
in soil, so his laboratory was well equipped to take data.

Several
physics majors helped with analyzing and plotting data, but
one physics major in particular, Lisa Pawelski, quickly assumed
a leadership position and directed much of the laboratory work.

The Dickinson
nuclear physics laboratory contained an 8.5% efficient Ge(Li)
gamma radiation detector connected to a Northern 1024 Multichannel
Analyzer with a TTY output. Because the system did not have
a plotter, student volunteers made the graphs of counts vs.
energy by hand. The samples were massed and placed in Marinelli
Beakers for more efficient counting. The standard count time
was 20000 seconds (5.5 hours), although a few samples were
counted for shorter or longer times. Based on the area of
the sample and the fraction of the sample counted, the results
of any activity detected were reported in pCi per square foot.
(A picoCurie (pCi) is 0.037 decays per second, or one decay
every 27 seconds. A pCi is a small activity; the human body
contains 1 million pCi of natural radioactive potassium-40)

In addition,
the researchers collected and counted some rainwater samples.
These were from a downspout at Luetzelschwab's house. These
results are reported in pCi per liter.

Anybody
collecting samples around TMI carried a radiation meter to
measure the radiation dose rate at the sampling location.
However, not all of these readings were recorded, mostly because
they were just normal background radiation, however some may
have just been forgotten.

At 7:30
on Thursday morning, Luetzelschwab collected a soil sample
from his garden. (His house was 2 miles west of TMI, which
happened to be downwind for more than a day after the accident.)
This sample was 1.5 square foot of soil to a depth of 5/8
to 3/4 of an inch. This sample was designated A-1 (the first
sample taken at site A). This sample surprisingly contained
radioactive xenon gas (Xe-133 and Xe-135). Noble gasses do
not react with anything, so they were not expected to be there.
However, the night before some rain fell in the area and xenon
is very soluble in water, so the rain washed the xenon out
of the air and deposited it in the soil. The calculated activities
of Xe-133 and Xe-135 are shown in Table 2. Based on the reported
rainfall (0.04 inches) and the xenon activity detected in
the soil sample, they calculated the activity in the water;
these results are in Table 3. The one other radioactive material
of concern was Iodine-131; none was detected in this sample.

The next
day Luetzelschwab collected a second sample from his garden.
Then he set up a series of sampling locations that surrounded
TMI. The map in Figure 1 shows these locations where he gathered
samples over the next three weeks and Figure 2 shows the wind
direction at the time of sampling and the rainfall (as reported
by the National Weather Bureau). Generally, samples were collected
at downwind locations. Table 1 gives the date and sampling
times for each location. Tables 2 and 3 give the results of
the analyses of these samples. Only samples from location
A gave any detectable levels of radioactivity from TMI. No
sample showed any I-131 even though the TMI scientists reported
some (15 to 30 Ci) was released. (Luetzelschwab used this
same sampling/detecting system in October, 1976, to detect
fallout from the Chinese atmospheric nuclear weapons test.
He readily detected I-131 in these samples.) Table 4 gives
the minimum detectable activity. Table 5 shows the measured
radiation dose rates that were above background levels.

Concurrently
with the soil sampling program, Laws set up a radiation monitoring
program at Dickinson (about 20 miles west of TMI). Every few
hours, students recorded counts from a Geiger Mueller Tube.
Over the two weeks of measuring, the radiation levels never
rose above normal background, except after a rain. Rain washed
radon progeny out of the air and the decay of these radioactive
progeny caused an increase in the count rate.

Figure 1

Map Showing Sample
Locations

Figure 2

Wind Direction and
Sample Collections

*A
Roentgen (R) is a measure of exposure to gamma radiation.
An mR is one-thousandths of a Roentgen (0.001 R). Normal gamma
radiation background from radioactivity in the soil produces
20 to 40 mR of exposure to most every person in the world.
See Chapters 1, 2, and 3 in "The
3 R's; Radiation, Risk, and Reason" for a full explanation
of the units of radiation dose and exposure.